Superantigens (SAgs) causes many human diseases. Although much is known about the structure and function of the pyrogenic SAgs encoded by staphylococcal and streptococcal bacteria, far less is known about the non-pyrogenic SAgs such as arthritis-related Mycoplasma arthritidis mitogen (MAM) and Crohn's disease-associated protein 12 (PfiT). MAM is structurally, and possibly functionally, distinct from other SAgs. In contrast to other SAgs, MAM was found to directly interact with not only TCR V0 but TCR Va. However, whether and how TCR Va affects MAM recognition is not known. 12 and its full-length protein PfiT encoded by Pseudomonas fluorescens are novel enteric SAgs. 12 and PfiT are important pathogenic factors and serological markers of Crohn's disease, an inflammatory bowel disease. However, although both MAM and PfiT play important roles in autoimmune diseases of inflammatory nature, how MAM and PfiT perform their biological function and their roles in inflammatory diseases are not known. Understanding how pathogenic factors such as MAM and PfiT are recognized by host receptors is key to resolve these issues and is fundamental for our understanding of disease pathogenesis. Our long-range goal is to determine how SAgs are recognized by host receptors. The objective of this application is to determine the structures and functions of MAM and PfiT. The specific aims are: 1) Determination of the molecular mechanism of MAM recognition by TCR. Crystal structure of a ternary complex between pMHC, MAM, and TCR derived from CD4+ T cells will be determined and compared with a ternary complex with a TCR derived from CD8+ T cells. These structures should resolve how TCRs derived from both CD4+ and CD8+ T cells recognize MAM as well as how TCR a chains affect their interactions with MAM. Mutagenesis, genetic, biochemical, and immunological approaches will then be used to determine the role of TCR Va in MAM recognition. 2) Determination of the structure and function of the Crohn's disease-assocaited SAg PfiT. Crystal structures of PfiT and its close homolog PA2885 will be determined and compared. The human receptors (TCR and pMHC) will be determined. The affinity and kinetic parameters of PfiT binding to TCR and to pMHC will then be determined, followed by determination of structures of PfiT in complex with host receptors-TCR and pMHC. The sum of these studies will contribute to our understanding of disease pathogenesis, and will provide new avenues for therapeutic intervention.